This is a study of the possible relationship between the peculiar cation selective properties in the conduction of certain phospholipid membranes and the unique cation requirements of Na-K-ATP-ase. Certain phospholipid membranes containing cardiolipin conduct K ion much more readily than Na ion, and their pattern of conduction and inhibition by cations parallels the enhancement and the blocking of the phosphatase step of ATP-ase. Cardiolipin can participate in the binding of ATP to the enzyme. This relationship will be studied by adapting the information on ion probes of phospholipid channels to the reported steps in the Na-K-ATP-ase mechanism. Also, because of cardiolipin's unique electrolytic properties in vitro, a survey will be made of the cardiolipin content and of the Na-K-ATP-ase activity of the microsomes of a variety of tissues in search of a correlation.